My answer to the original question would be either the M&P 5.7 (5.7x28mm) or the CZ Shadow 2 Compact (9mm) depending on your caliber preferences. Both can be true "1 gun solutions" and offer unique blends of functionality, shootability, and versatility (e.g., can be used for carry, home defense, and competition). The M&P 5.7 also offering the fourth element of affordability..
I recently traded in my FN 510 (10mm) for the new M&P 5.7 (5.7x28mm) as my griz gun. As much as I shot that FN 510, I just struggled to shot it accurately, especially after that first initial shot. I've looked into the 5.7x28mm round a couple times over the years but always came away unconvinced given terribly expensive ammonition prices and very poor bullet options from factory ammo. That changed however when Hornady jumped on board late last year with the Critical Defense line in 5.7x28mm. Concurrently, practice ammo prices have dive bombed and you can regularily find FN branded practice ammo with the 40gr V-Max in the $0.40 range.
I absolutely love this gun! 22+1 capacity, extremely shooter friendly ergonomics, and with the new Tempo (gas) operating system Smith & Wesson designed on this gun, it has literally zero recoil! It's hard to believe it when you shot this gun.. I'd dare say the recoil is less than my FN 522 (full size 22 pistol) despite it being a lighter gun pushing a much higher horsepower cartridge. I'm pushing 40 gr critical defense loads from Hornady at nearly 1875 fps per my garmin xero. I've never felt more capable with a handgun in my hands in my life.. It's stupid accurate, extremely fast (gas operating system basically eleminates all muzzle rise), plenty capable from a destruction standpoint assuming the correct bullet selection, light and slim for carry if wanted, and well priced. I still have a couple 9mm guns including my beloved CZ 75; however, if push came to shove and I could only have one, I think I'd grab my M&P 5.7.
The one 9mm I would consider as a "1 gun solution" over my M&P 5.7 would be a CZ Shadow 2 Compact. I was shocked at how fast/accurate it was for a 9mm compact, but at $1200 or more, it's just not worth it for me at this point..
